Trailing and reeling cables are electrical power cables for mobile apparatus, such as large excavators, draglines, stackers and reclaimers. The cable allows the mobile apparatus to move without disconnecting its electric power supply.
A reeling cable is specifically designed to be frequently reeled on and off a cable drum or reeler (e.g. an iron ore rail-mounted reclaimer), whereas a trailing cable is specifically designed to be moved with the mobile apparatus (e.g. a coal mine dragline, mineral sands mobile processing plant).
Requirements for trailing and reeling cables
Trailing and reeling cables pose an elevated safety risk (e.g. a damaged trailing cable can lead to earthing faults and workers being exposed to direct and indirect electric shock hazards). They require specific design mitigation measures, which are detailed in the Work Health and Safety (Mines) Regulations 2022 (WHS Mines Regulations).
Design standards applicable to trailing and reeling cables
Trailing and reeling cables must conform to Australian Standard AS/NZS 2802:2000 Electric cables — Reeling and trailing — For mining and general use (other than underground coal mining) and incorporate a pilot core arranged to cut off the supply of electricity in the event of a break in the earthing circuit.
Once an earth continuity relay and associated connections of the protection system are installed, the pilot core allows pilot earth loop or earth continuity monitoring protection to function.
AS/NZS 2802:2000 (Foreword) does allow for the consideration of AS/NZS 1802:2018 type cables which may be suitable for installation in underground metalliferous mines. However, if a Type 241.1 trailing cable is to be installed in a metalliferous underground mine, a risk assessment on the cable's suitability needs to be completed by a competent person, taking into consideration any advice on suitability from the cable supplier.
Refer to r. 641E of the WHS Mines Regulations for more details.
Why is a pilot core included?
The inclusion of a pilot core allows pilot earth loop or earth continuity monitoring protection to function. It is important to realise, though, that this protection method is not the same as pilot wire differential protection (where the current entering and leaving a cable is measured and compared).
Rather, earth continuity protection functions by establishing a loop current through the pilot core, returning through the cable earth. This is typically achieved using an earth continuity protection relay to apply a low voltage DC signal between the pilot and earth of the cable. By measuring this signal, the resistance of the circuit can be determined and protection operation initiated for variance from a pre-tested value. This variance is indicative of a break in the earth return path of the cable, of the cable itself, or a deterioration of the earth resistance that can lead to indirect contact electric shock hazards. With this arrangement, an end-of-line resistor or diode is required to avoid a short circuit between the pilot cable and the earth.
Functions required for the earth continuity protection system
For the overall earth continuity protection arrangement, the system must perform the functions described below.
- The protection system must monitor the status of the earth continuity for any break in the earth return path. Typically, this will require a status indication of earth continuity healthy, open-circuit or short-circuit.
- In the event that the status of the pilot earth continuity circuit is open-circuit or short-circuit, an interlock must immediately initiate automatic de-energisation of the mobile apparatus.
- The protection system must also monitor the resistance of the pilot-earth loop. If the earth resistance exceeds the pre-tested value that maintains indirect contact at a safe level, an interlock must initiate automatic de-energisation of the mobile apparatus.
- When a fault does occur, the system should require manual reset — the system should latch and thereby store the fault condition in order to prompt fault-finding and testing.
- The protection system should ideally interface with the control system of the mobile apparatus such that a healthy status indication for the pilot cable is required prior to start.
Commissioning and testing required at installation
Earth continuity protection must be installed, commissioned and tested in accordance with the manufacturer’s instructions and recommendations. These instructions and recommendations are required to conform to Australian Standard AS 1747:2003 Reeling, trailing and feeder cables used for mining — Repair, testing and fitting of accessories.
Refer to r. 641E of the WHS Mines Regulations for more details.
Records and details of electrical installing work carried out at the mine are required to be recorded in the electrical logbook. This includes commissioning check sheets and test records, which must be kept. An entry should be made in the logbook indicating where the sheets and records can be found.
The earth continuity protection relay itself should have a label affixed indicating the test date and who conducted the tests, including company details.
Refer to r. 641C of the WHS Mines Regulations for more details.
Safe work practices
Trailing and reeling cables need to be installed, located, supported and protected in a way that:
- minimises the risk of damage to the cable and to any connecting or coupling device
- does not obstruct any access way (this may include a pit roadway)
- separates the cable from other services at the mine.
In practice, this may require the use of protective berms and the use of visible markers for trailing cables.
Refer to r. 641E of the WHS Mines Regulations for more details.
Ongoing maintenance
Mines (including exploration sites) are required to have an appropriate maintenance system in place to ensure that electrical equipment and installations are maintained in safe working order.
A routine maintenance system must be implemented for testing earth continuity protection systems to confirm the integrity of the system and the pre-tested pilot and earth loop resistance value. This is critical because the system relies on the accuracy of this pre-tested value.
For surface operations, routine testing should be done every three months. For quarries, dredges (other than a floating treatment plant) and underground mines, this test must be done every month.
Refer to r. 641F of the WHS Mines Regulations more details.
Resources and guidance
- Managing electrical risks in the workplace: Code of practice
This Code provides practical guidance on managing electrical risks in the workplace. It applies to all workplaces where a PCBU has management or control of electrical equipment, including electrical installations, or carries out electrical work on or near energised electrical equipment, including electrical installations.